Mode suppression in resonant cavities



June 14, 1955 w. A. EDSON MODE SUPPRESSION IN RESONANT CAVITIES FiledSept. 26. 12947 Fla. 4

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I flvan CIRCUMFE'RENC! or perm or at noon) fiIEEP cnoovc 71' II?! MODEPISTON 54c: WIT/1' A DEEP anoovs- INVENTO/P WA .EOSON ATTORNEY UnitedStates Patent MODE SUPPRESSION IN RESONANT CAVITIES William A. Edson,Atlanta, Ga., assignor to Bell Telephone Laboratories, Incorporated, NewYork, N. Y., a corporation of New York Application September 26, 1947,Serial No. 776,194

9 Claims. Cl. 333-83 This invention relates to resonance chambers formicrowaves and more particularly to the suppression of extraneous modesof oscillation in such chambers.

An object of the invention is to trap and absorb extraneous modes havinga radial field component by means of terminated, impedance transformingdeep grooves.

Another object of the invention is to prevent interaction between thefront and back cavity of an echo box or tunable resonance chamber bymeans of absorption loading in the form of a sealing ring on theperiphery of the tuning piston.

A feature of the invention is a mode suppressor for a resonance chamber,characterized by terminated deep grooves located therein, the groovesbeing concentrically arranged and having a depth related to where /\g isthe guide wavelength therein appropriate to an undesired mode, wherebynumerous modes of initial intensity comparable to the main mode, may beeffectively degraded.

Another feature of the invention is a reflecting tuning piston having anelectromagnetic sealing ring on the periphery thereof to provide highloss for undesired modes and to prevent interaction between the frontand back cavity in a tunable resonance chamber.

Extraneous mode suppression characterized by shallow, concentric cuts ina conductive end plate of a resonance chamber for discriminating againstundesired modes having a radial field component, have heretofore beendisclosed in the United States application Serial No. 570,192, filedDecember 28, 1944 by W. A. Edson, now patented as United States Patent2,698,923, January 4, 1955.

More effective mode suppression than that obtainable by shallow cuts hasbecome important particularly in the centimeter or millimeter waverange, where the number of extraneous modes present increases and theinitial intensity thereof approaches that of the operating mode.

In accordance with the invention, deep grooves are provided in thereflecting face of a tuning piston, the grooves being concentricallyarranged and having a depth related to A lossy termination is providedat the base of the deep grooves, where undesired modes having a radialcomponent of field distribution may be fully absorbed. The tuning pistonis also provided with peripheral loading, such as a ring of absorptionmaterial, for example neoprene. The piston loading may be viewed as asealing ring, preventing electromagnetic leakage between the activefront and the back cavity of the resonance chamice ber or echo box,thereby isolating said cavities from interaction effects.

In the drawings:

Fig. 1 represents a resonance chamber and the tuning piston end portionthereof; 9

Fig. 2A shows the folded deep groove construction in said piston;

Fig. 2B is a modified deep groove construction;

Fig. 3 shows a front view of a piston with a single deep groove forsuppressing the TElln mode family; and

Fig. 4 shows the equivalent rectangular wave guide analogue of the deepgroove shown in Fig. 3.

Referring to Fig. 1, a tunable resonance chamber 1 of cylindrical form,having a highly conductive interior coating of silver or the like, isprovided with a movable tuning piston 2 adapted to be reciprocated by amechanical drive which may be of a type such as disclosed in the UnitedStates application of W. F. Kannenberg et al., Serial No. 544,990, filedJuly 14, 1944, now Patent No. 2,537,139 issued Jan. 9, 1951, or thelike, now patented as United States Patent 2,537,139, January 9, 1951.

The resonance chamber 1 comprises an end base plate 3 on which isaccurately positioned the right circular cylinder 4 having transverseperpendicular supporting or aligning flange 5, so that the principalaxis of the cylinder is ideally perpendicular to the base plate 3 at itscenter.

The end plate 3 may be provided with a pair of slits and a TEos modesuppressor as more fully disclosed in the United States application ofR. W. Lange, Serial No. 770,988 filed August 28, 1947, now patented asUnited States Patent 2,701,343, February 1, 1955. Between the base plate3 and the flange 5 of cylinder 4, a loading ring 6 of neoprene absorberis seated in a deep groove 6', approximately in depth, where A is thewavelength in the wave guide. The continuity of conductive surfacebetween the end plate and the side wall of the chamber is therebyinterrupted by the deep groove 6', which traps and absorbs the energy ofextraneous modes propagating over said surfaces.

The tuning piston 2 is provided in its reflecting face 2' with aplurality of cylindrically shaped, deep grooves in depth for thecorresponding mode or modes, and

Y are each terminated by a loading dielectric ring 10 made of conductiveneoprene or the like, extending trans versely to the principal axis ofthe resonance chamber 1. To produce and maintain perfect parallelismbetween the piston 2 and base plate 3, pivoted centering bolts 11 asdisclosed in the United States application of R. W. Marshall, Serial No.714,621 filed December 6, 1946, which issued as United StatesPatent2,587,055, February 26, 1952, are provided on the rear surface of thepiston.

The central bolt 11' has its head 21 recessed in the piston, providing aflush surface with the piston face 2'. The deep groove 7 is formed bythe hollow space, bounded by the cylindrical side surface of head 21 andthe adjacent, recessed cylindrical surface, transverse to the pistonface 2. The rear end of the head 21 is in conductive contact with therear of the piston.

The electric field of the TEOln operating mode, being everywherecircular, fringes into the concentric groove and since the neopreneabsorption rings 10 are recessed sufiiciently .deep, the loss to thewanted mode TEOln is consequently very small. The depth of theconcentric grooves for this purpose is The depth provides effectively aquarter wavelength transformer, whereby the impedance at the piston face2 matches the impedance provided by the lossy terminations or rings 10at the base of the grooves. When the proportions of air space andneoprene loading are proper in the deep grooves 7, 8, 9, unwanted modeshaving a for an undesired mode.

radial component of the electric field, are severely degraded as theradial currents thereof flow across the circular zones of the deepgroves.

As the requirements of echo boxes advance to the higher frequencyregions (such as the millimeter and the l to 10 centimeters wavelengthregion), and higher Q, the total number of modes possible in the workingfront cavity 15, the back cavity 16 and in between the two increases.These resonances can produce spurious ring and transmission andinteraction effects comparable to those at a mode crossing. Therefore itbecomes increasingly important in these frequency regions to eliminateall back cavity effects. For this purpose, a piston loading or sealingring 19 operates in the peripheral gap space 20 to add loss, eliminateback cavity effects and furnish additional mode suppression in theactive cavity, through the operation of a cylindrical deep groove havingan open rather than a short-circuited end. The piston skirt 18, thatportion of the piston adjacent and parallel to the side wall of thechamber 1 and measured back from the active piston face, supports a band19 in width of lossy dielectric, such as conductive neoprene. The widthof the dielectric and its location in overall depth, having an airfilling deep communicating with the front cavity and a dielectricfilling 10' of conductive neoprene in depth, acting as an'absorbingtermination for unwanted modes.

Factors in design of a deep groove Fig. 3 shows schematically the pistonface with a single deep groove therein, located at radius r, and havinga width w.

The factors which enter into the design of individual deep grooves forsuppression .of specific unwanted modes (all modes except TEU,m,n) arethe radius r, the depth and width w of the groove, and the properties ofthe dielectric loading material as illustrated in Figs. 2A, 2B, 3.

Inasmuch .as the .deep groove depends in part for its discriminationupon the direction of current flow of desired and undesired modes, it isbest located in regions of maximum radial current of the undesiredmodes. The degradation of the unwanted mode is proportional to thedissipation in the lossy dielectric loading and this loss isproportional to the square of the radial current and to a resistanceterm. The resistance term is a minimum for a groove located at zeroradius and a maximum at the chamber periphery and for this purpose istaken as varying with radius. In a simple case where the resistancevaries linearly with the radius, the relative effectiveness of the deepgroove is proportional to l r where I is the relative intensity ofradial current and r is average relative radius.

For better visualization of its electrical characteristics, a deepgroove may be considered as an imaginary rectangular wave guide foldedaround the axis of the piston until its opposite sides become adjacentand are removed, see Figs. 3 and 4. Analogously, the resonance chamber 1for the purpose of this analysis, may be regarded as replaced by arectangular guide, supporting the same operating mode and having thesame width as the rectangular guide of Fig. 4. The height of the formerequivalent guide will hereinafter be termed the full height todistinguish it from the height of the previous rectangular guide (Fig.4).

Referring to Fig. 4, the length dimension 15 of the equivalentrectangular wave guide would correspond to the depth of the deep groove,measured along the principal axis from the piston face 2' to the bottomof the respective groove. The rectangles width 12, which determines thefrequency of cut-off, corresponds to the mean circumference of the deepgroove, and its height 13 corresponds to the radial measure of thegrooves width. For the mode family TE1,m,n in the resonant chamber 4,the field distribution in the deep groove considered in terms of anequivalent rectangular guide, will be that of the TE2,0 mode. i. e. theequivalent rectangular guide 4 supports a mode having two halfwavelengths measured along its width dimension 12 (Fig. 4). For theTE2,m,n family in the resonant chamber, there would be four halfwavelengths measured along the width in the equivalent rectangular guideof the deep groove.

The cut-off frequency fc of this rectangular guide is and the guidewavelength (in the absence of a loading dielectric) is:

It is to be noted that the equivalent guide wavelength is a function ofthe radius r of the groove and the 1 index of the mode.

The effective electrical length of the air and lossy as determined inexperiments at l to 3 centimeters wavelength. The deep groove was foundto provide the best termination to the wave guide chamber 1 when theshortest air section, somewhat less than 4 and a conductive neoprenesection of approximately the same length were used. Sections longer byintegral numbers of half guide wavelength were more critical indimensions and more frequency sensitive.

The impedance seen at the junction of the wave guide chamber and groove,looking into the groove, is the admittance of the loaded groove inparallel with a capacitive susceptance produced by the change in theirrespective heights. The best termination to the full height guide isobtained when the conductance offered by the groove matches that of themain guide and when the susceptance offered by the groove cancels thatproduced by the aforementioned change in height. It appears that theheavily loaded section slightly less than long presents a low resistivetermination to the unloaded section and this impedance transformed byless than a section presents high resistive and inductive components tomatch the large guide and neutralize the reactance due to the change inheight. The impedance looking into the loaded grooves depends on themode being suppressed, the dimensions of the groove, the conductivityand dielectric constant of the loading material.

The width w of the deep groove measured along a radius of the cavitycorresponding to the height 13 of the equivalent rectangular guide isimportant in two respects. First, the capacitive susceptance produced bythe change in height is dependent upon the width of the groove, which inturn reflects in the depth dimension as stated above. Second, thedegradation of the desired mode depends on the reflecting surface of thepiston that is removed, so

the width of the groove should be held to a minimum. 7

A width approximately 1% of the cavity diameter has been successfullyused in several designs of echo boxes.

Folded grooves In general the treatment of cylindrical (Fig. 2B) andfolded grooves (Fig. 2A) is similar. The fold is made in the unloadedsection or at the junction of the loaded and unloaded sectionprincipally to permit constructing the loading in the form of flatwashers. The washers can be made easily and can be retained in place sothat the construction will withstand shake and shock tests.

The computation of A is further complicated by the fold; however, thefolded grooves used in3 centimeters and 1.25 centimeters wavelengthbands were computed the same as cylindrical grooves. The depth of eachsection measured along the center line of the groove is slightly lessthan The width, measured along the radius in the cylindrical part andalong the bore axis in the folded part, is approximately 1% of thecavity diameter as in the cylindrical groove.

6- Suppression to a group of modes The suppression of a group of modesfollows the pattern described for the suppression of an individual mode.The best radius for a deep groove is selected for each mode and acompromise radius is chosen. In some cases a single radius may provesatisfactory for less than all of the modes of the group. This thenrequires a sorting of modes on the basis of optimum radius of groovesand the selection of two or more radii for the location of grooves. Inmaking this selection, consideration must be given to the relative valueof the unwanted modes and the relative suppression required. In general,greater suppression is required for those modes having the largervalues. 4

The modes to be suppressed by each groove may be collected into familiesaccording to the Z index and the depth of each groove computed for thesefamilies. A compromise in depth is made according to the relativesuppression required. Modes requiring the greatest suppression arefavored. If large values of l are encountered, as at the 1.25 centimeterwavelength band design, these modes must be treated by special groovesclose to or at the piston edge.

A particular example is the choice of the deep groove to suppress theTE1,3,n mode in the 1.25 centimeter wavelength echo box. A groove atradius chosen to suppress many other unwanted modes offered only a fewper cent relative suppression to the TE1,3,u mode as the best locationto suppress the TEl,3,n is at 15% radius. The echo box was again testedwith a piston incorporating a deep groove with mean radius at about 18%(a slight compromise for mechanical considerations), and the TE1,3,nmode was completely suppressed for both ring and transmission. The ringwas degraded from 6 /z s at least to 2 /2ns (as observed by thetransmitter pulse and receiver recovery) which with a decrement of about9 db/ s indicates a suppression exceeding 36 db. The suppression totransmission exceeds 25 U. H. F. db for the mode was tentativelyidentified by a slight trace (less than 4 meter division) of rectifiedcurrent with minimum U. H. F. attenuation in the transmission path.

In the measurements of a 1.25 centimeter Wavelength echo box simulatedin 3 centimeter band, absolute identification of modes was difficultparticularly if various perturbations were introduced by modesuppression devices. The deep groove performance was judged by thereduction in extraneous transmitting and ringing modes both as to totalquantity and value. At approximately 9420 megacycles a total of 15extraneous modes were reduced to 3 (excluding TEo,2 and 0, 3 modes)after installing a deep groove at 40% radius. At 9500 megacycles, 16extraneous modes were reduced to 7 and at 9360 megacycles 13 reduced to6. The greatest transmission reduction exceeded 40 db measured on atransmission oscilloscope at video frequencies. Extraneous rings, someas long as 3000 yards were reduced at least to 400 yards (the recoverytime of the radar). The introduction of the groove reduced the averagetransmission and ring of the wanted mode about 3 to 5%.

Piston peripheral loading There are two general factors in the design ofsuppression in the form of a lossy skirt at the piston gap, namely thedepth of loading measured along the axis and the width measuredradially.

The peripheral gap space can be considered an open end deep groove. Thelengths of the loaded and unloaded 7 portions are dependent on the 1index of the modes to be suppressed. The depth of the unloaded Sectionis slightly less than where )\g is computed as in the piston deep grooveand the depth of the loaded section is increased to approximately tocompensate for removing the closed end of the groove.

Unless suppression is required for a specific mode these depths aresometimes reduced as a compromise on the physical size of the piston forshake and shock considerations.

The width of the peripheral gap is primarily selected to perturb the TMcompanion of the wanted mode, consistent with a small degradation of theTE mode. The width is about 3 to 5% of the diameter. The gap widthapplies at the piston face and may change along the piston skirt. Asstated previously, changes in width affect the equivalent lengths of theunloaded and loaded sections of the gap.

There must be sufiicient volume of lossy material on the skirt tosuppress transmission to the back cavity adequately. This was achievedin the 1.25 centimeter echo box range in a reasonable length byincreasing the gap beyond the unloaded section to accommodate aconductive neoprene band. From mechanical considerations this step ingap Width provides a locating ridge for the band and is sutficient toprevent the loading material from touching the cavity wall. For aparticular conductive neoprene used, the thickness of materialis .040inch for 1.25 centimeter wavelength and inch in the 3 centimeter region.The surface of the band extends slightly above the unloaded gap section.

It should be understood that the deep groove mode suppressors previouslydisclosed as located in the piston face of the echo box or resonantchamber, may be also situated in the fixed end plate thereof or in theside walls without departing from the spirit of the invention. Also itis possible and in some cases mechanically desirable, to locate some ofthe deep grooves in the piston face and some in the end plate withoutdeparting from the spirit of this invention.

What is claimed is:

1. In a substantially closed electromagnetic resonator tunable about apredetermined operating frequency and having substantially the shape ofa right circular cylinder, a conductive piston movable axially of thesaid resonator and dividing the said resonator into an active resonatingchamber adapted to support electromagnetic waves of the operatingfrequency in a TEUI mode, said piston having two cylindrical peripheralsurfaces each spaced from the cylindrical wall of the resonator, one ofsaid surfaces having an axial length of approximately a quarterwavelength and the other of said surfaces carrying a band of lossymaterial having an axial length of approximately a half wavelength, saidpiston having, in the face thereof that bounds said active chamber, atleast one annular recess concentric with said chamber, said recesshaving a low-loss impedance matching section that is substantially aquarter wavelength in depth terminated in another portion containinglossy material wherein the wavelength taken corresponds to that of anundesired mode.

2. In combination, a cylindrical hollow cavity resonator adapted tooperate in TEDmn mode and having a tuning piston, the active face ofsaid piston being provided with a series of selective annular deepgrooves located in regions of maximum radial current vof unclesiredmodes, said grooves adapted to provide an impedance match between theimpedances at opposite ends thereof and the depth thereof beingsubstantially equal to an integral multiple of a quarter wavelengthcorrespending to the Z index of a family of undesired modes, and anabsorber located at the base of each groove to attenuate the undesiredmodes without substantially affecting the operating mode.

3. In combination, a hollow cavity resonator, means for exciting saidresonator with electromagnetic energy in a predetermined mode ofoscillation, a tuning piston for said resonator having two cylindricalperipheral surfaces, one thereof having a groove and having an axiallength of approximately a half wavelength of an undesired mode, theother surface having an axial length of approximately a quarterwavelength for impedance transformation, and a band of lossy dielectricfilling said groove.

4. A hollow cavity resonator having conductive end walls, means forexciting said resonator with electromag netic oscillations in a TEOmnmode, one end wall being provided with spaced concentric deep groovescommunicating with .the cavity space, said grooves each having anextraneous mode absorber at the base thereof, the depth of said groovesbeing different and substantially equal to an integral multiple of aquarter wavelength corresponding to the extraneous modes fortransforming the impedance of said resonator to the absorber impedance.

5. The structure of c1aim,4 wherein said grooves are filled in theirbasal portions to half their depth with said absorber.

6. The structure of claim 2 wherein each of said grooves has atransverse fold forming its closed end and said absorber is a flat ringin width filling the closed end of said groove, where A is thewavelength corresponding to an extraneous mode.

7.. A hollow cavity resonator comprising conductive walls, means forexciting said resonator electromagnetically in a predetermined operatingmode and extraneous mode selective devices comprising concentric deepgrooves of unequal depth corresponding to the Z index of extraneousmodes suppressed thereby, said grooves being located in a conductivewall, said grooves communicating with the cavity space at one endthereof and being loaded with dissipative material at the other end,said grooves being located radially at positions where Ip l is maximumwhere I]; is the relative intensity of radial current of undesired modesand r is the average relative radius.

8. The structure of claim 7, wherein said wall comprises a tuning pistonhaving two cylindrical peripheral surfaces, one thereof being a groovesubstantially a half wavelength long corresponding to an undesired modeand a band of lossy dielectric filling said groove.

9. The structure of claim 2, wherein said resonator is provided with arecessed end plate, the recess being offset and having an absorbertherein for trapping extraneous modes propagating between the end plateand cylindrical side wall thereof.

References Cited in the file of this patent UNITED STATES PATENTS2,151,118 King Mar. 21, 1939 2,197,122 Bowen Apr. 16, 1940 2,253,589Southworth Aug. 26, 1941 2,415,962 Okress Feb. 18, 1947 2,423,396 LinderJuly 1, 1947 2,439,388 Hansen L Apr. 13, 1948 2,465,719 Fernsler Mar.29, 1949 2,484,822 Gould Oct. 18, .1949

